Make log format more customizable

[vjik]

1. Allow to set own callable for convert value to string (https://github.com/yiisoft/log/blob/master/src/Message/Formatter.php#L257)
2. Add template (and allow user to change it) for context (https://github.com/yiisoft/log/blob/e0551896a54a91dd3c15d415167d0402c7bf9aaf/src/Message/Formatter.php#L196C22-L196C32). It should be possible to change the order of output trace, common context, message context.